1. Technical Field
The present invention relates to a liquid crystal device, a pixel circuit, an active matrix substrate, and an electronic apparatus.
2. Related Art
A reflective liquid crystal device is, for example, installed in an electronic apparatus, such as a cellular phone terminal, a notebook personal computer or a reflective projector. The reflective liquid crystal device is configured so that a liquid crystal layer is held between a glass or silicon substrate, or the like, that is provided with, for example, data lines, scanning lines, switching elements such as transistors, electric charge storage capacitors, and reflective pixel electrodes formed of aluminum, or the like, and a glass substrate, or the like, that is provided with an opposite electrode, and the like, formed of a transparent conductive film. Because the pixel electrodes are of a reflective type, the switching elements, such as transistors, may be provided under the pixel electrodes. Thus, even when a resolution is increased, the aperture ratio of the panel does not decrease and, therefore, it is relatively easy to achieve both a high resolution and a high luminance.
However, when an analog pixel circuit that holds a pixel voltage by a holding capacitor is used, the voltage value of the holding capacitor decreases with time, so that the lightness and/or contrast of a display image may vary.
To solve the above problem, a liquid crystal device has been proposed in which a 1-bit memory cell is arranged under the reflective pixel electrode of each pixel (which is, for example, described in JP-A-B-286170). In the liquid crystal device that includes such a memory cell in each pixel, an image signal supplied from a corresponding one of data lines is latched by the memory cell of each pixel and the signal is then applied to the liquid crystal layer of the pixel. Each of the memory cells holds the preceding signal until a new signal is written thereto. In this manner, for example, after a still image has been saved in the memory, another still image is displayed, and thereafter the saved still image is display again. The thus display switching may be performed easily and efficiently. In addition, by digitizing a pixel voltage, it is possible to obtain such an advantageous effect that degradation of display quality due to crosstalk, or the like, hardly occurs.
In the meantime, in order to prevent the occurrence of a so-called burn-in (a phenomenon in which a display image degrades because liquid crystal molecules are aligned in the same specific direction) due to a direct-current voltage applied to the liquid crystal, it is effective that the polarity of a voltage applied to the liquid crystal is inverted periodically (which is, for example, described in JP-A-5-303077).
Meanwhile, in a liquid crystal device that includes a memory cell in each pixel, the configuration of a circuit that inverts a voltage applied to the liquid crystal is, for example, described in JP-A-2005-148453 and JP-A-2005-25048. The technologies described in these publications are the same in that the polarity of a voltage applied to one of electrodes of the liquid crystal and a voltage applied to an opposite electrode (common electrode) is inverted periodically. Note that, in the technology described in JP-A-2005-148453, the supply of complementary signals acquired from an SRAM to the liquid crystal is switched by turning on/off of transistors. On the other hand, in the technology described in JP-A-2005-25048, when an offset occurs at the time when a voltage applied to the liquid crystal is inverted, it causes a burn-in. Thus, an offset voltage of the voltage applied to the opposite electrode (common electrode) fine adjusted so that a response waveform acquired from an optical sensor becomes the same in every field.
Another example of a liquid crystal device has been known, in which alignment of liquid crystal molecules is controlled by applying a liquid crystal layer with an electric field in a direction of the substrate plane (hereinafter, referred to as “lateral electric field mode”), and, depending on the form of electrodes that apply an electric field to the liquid crystal, it may be called an IPS (In-Plane Switching) mode, an FFS (Fringe-Field Switching) mode, or the like (which is, for example, described in JP-A-2001-337339). The lateral electric field mode liquid crystal controls the state of transmission of light by rotating horizontally aligned liquid crystal molecules in a lateral direction. Because the liquid crystal molecules are never inclined in a vertical direction, variation in luminance and/or variation in color depending on a viewing angle are small. Thus, the lateral electric field mode liquid crystal is used when a high viewing angle characteristic and a high-quality color developing property are required.
To prevent burn-in of a liquid crystal, it is necessary to prevent a direct-current voltage from being applied to the liquid crystal over a long period of time. FIG. 18A and FIG. 18B are views that show the operation necessary to prevent the occurrence of burn-in in a liquid crystal device, in which FIG. 18A is a view that shows the operation when a voltage is applied to a liquid crystal, and FIG. 18B is a view that shows the operation when no voltage is applied to the liquid crystal. In FIG. 18A and FIG. 18B, a liquid crystal of a type in which an electric field is applied to a liquid crystal layer vertically to the substrate plane (for example, a TN liquid crystal) is used.
As shown in FIG. 18A, when a voltage is applied to a liquid crystal 400, the polarity of a voltage applied to the liquid crystal is, for example, periodically inverted in order to prevent the occurrence of burn-in. That is, the polarity of a voltage applied to the terminals X1 and X2 shown in the drawing is switched periodically. Note that the liquid crystal 400 includes a lower electrode Lp and an upper electrode (common electrode) LCcom.
In addition, as shown in FIG. 18B, in order to prevent the occurrence of burn-in when no voltage is applied to the liquid crystal 400, it is important not to produce a direct-current offset in such a manner that the lower electrode Lp and the upper electrode (common electrode) LCcom are short-circuited to be equipotential. Note that, in FIG. 18B, for the sake of convenience, the electrodes of the liquid crystal is short-circuited using a switch SW1; however, the short circuit of the electrodes of the liquid crystal 400 is actually performed by applying the same voltage to each of the electrodes.
However, in the liquid crystal device that includes a memory circuit in each pixel, it is actually difficult to perform the ideal operation described schematically in FIG. 18A and FIG. 18S (operation to invert polarity and operation to short-circuit the electrodes for preventing burn-in).
FIG. 19A to FIG. 19C are views that illustrate problems, in a liquid crystal device that includes a memory circuit in each pixel circuit, when voltages applied to the electrodes of a liquid crystal are inverted.
The mode in which voltages applied to the electrodes of the liquid crystal are inverted includes a method, as shown in FIG. 19A, in which the voltage (Vcom) applied to the opposite electrode (common electrode) LCcom is fixed and the polarity of the voltage (Vp) applied to the lower electrode Lp is inverted, and a method, as shown in FIG. 19B, in which the voltage (Vp) applied to the lower electrode Lp and the voltage (Vcom) applied to the common electrode LCcom are interchanged at the same time. Note that, in FIG. 19A to FIG. 19C, voltages applied to the liquid crystal are respectively set to “5 V” and “0 V”.
When the method shown in FIG. 19A is employed, it is convenient because it is not necessary to change the voltage (Vcom=0 V) applied to the opposite electrode (common electrode) LCcom; however, the voltage (Vp) applied to the lower electrode Lp needs to be changed relative to the voltage Vcom. As a result, it is required to use a negative power supply. It is unrealistic to operate the memory circuit provided in each of the pixels with a negative power supply, so that the method shown in FIG. 19A cannot be employed for the liquid crystal device that uses the memory circuit.
Then, as shown in FIG. 19B, there is no other choice but to employ the method in which the voltage (Vp) applied to the lower electrode Lp and the voltage (Vcom) applied to the common electrode LCcom are interchanged at the same time. In this case, the problem is that, because the opposite electrode (common electrode) LCcom is an electrode that is shared by all the pixels of the liquid crystal device, the entire liquid crystal layer held between the substrates functions as a load capacity and, therefore, a change in voltage will be slow.
That is, as shown in FIG. 19C, in regard to the lower electrode Lp, a load is small because the electrode corresponds to one pixel. Thus, when the voltages applied to both electrodes of the liquid crystal are inverted (at time t1), the voltage (Vp) applied to the lower electrode Lp quickly changes. In contrast, the voltage (Vcom) applied to the opposite electrode (common electrode) LCcom changes slowly because of a heavy load and, as shown in FIG. 19C, a voltage is switched over a transition period T1 (from time t1 to time t2). Accordingly, in the transition period T1, a voltage applied to the liquid crystal gradually changes with time, and a change in transmittance ratio of the liquid crystal in accordance with the change in voltage is recognizable because of the slow change. Thus, a flicker (visual flickering) is likely to occur.
In addition, in order to perform the voltage inversion control shown in FIG. 19B, it is necessary to separately control the voltage Vp and the voltage Vcom by separate control circuits, so that a circuit configuration becomes complex.
FIG. 20A and FIG. 20B are views that illustrate problems, in the liquid crystal device that includes a memory circuit in each pixel circuit, when the electrodes of a liquid crystal are short-circuited (the same electric potential). As shown in FIG. 20A, both electrodes (Lp and LCcom) of the liquid crystal 400 are applied with ground electric potentials (GND1, GND2) from separate circuits (lines). However, the ground electric potentials (GND1, GND2) applied to the electrodes through the separate circuits (lines) may have a relative difference therebetween because a variation in voltage level occurs independently of each other.
Moreover, because the electrodes (Lp and LCcom) of the liquid crystal each have a two-dimensional area and, therefore, their voltages (Vp and Vcom) scatter in the planes of the electrodes. Thus, there is a possibility that a direct-current offset will occur in the electrodes of each pixel.
Accordingly, as shown in FIG. 20B, there is a possibility that a direct-current offset voltage (ΔV) will occur at the electrodes of each pixel of the liquid crystal 400. Note that Vgnd1 and Vgnd2 in the drawing represent voltages of both electrodes of each pixel by taking a voltage scatter in the planes of the electrodes into consideration. Such a direct-current offset voltage ΔV may cause a burn-in.
As described above, in the liquid crystal device that includes a memory circuit in each pixel, it is difficult to perform inversion of an applied voltage for preventing burn-in without occurrence of a flicker and to realize a complete short circuit that does not produce a direct-current offset. In addition, because it is necessary to separately control the voltages applied to the electrodes (Lp and LCcom) of the liquid crystal, a circuit configuration for control becomes complex.
Furthermore, the method to write image data includes a line sequential driving method in which image data are sequentially written to each of the pixel circuits connected to one scanning line and, at the time when writing to all the pixel circuits has been completed, the image data written to each of the pixel circuits are displayed by the liquid crystal, and a frame sequential driving method in which the operation to sequentially write image data to each of the pixel electrodes connected to one scanning line is sequentially performed for the number of scanning lines and, at the time when writing to all the pixel circuits has been completed, the image data written to each of the pixel circuits are displayed by the liquid crystal. However, even with any one of the methods, writing the image data to the pixel circuits will be reflected on the display screen and, therefore, it causes a flicker, or the like.